In the following, the operation of a color scanner will be described and its general functions and objectives are also explained.
In a color scanner, the color picture is placed on a rotatable scanning drum. A scanning device which is adapted to move axially with respect to the scanning drum scans the picture point by point and along the plurality of lines into which the picture is subdivided as it moves along. In the scanning device, the scanning ray is subdivided into three component rays. The component rays each have an associated color filter for separating the colors and opto-electrical transducers in order to produce three distinct signals.
The color signals, after they have been operated on by a logarithm operator, correspond to the scanned color density and are fed into a color operator in order to provide a color correction. The corrected color signals are fed into a recording unit. The recording unit includes a correcting stage for compensating the non-linearity and a recording lamp having a brightness or intensity modulated by the corrected color signals.
The recording lamps expose a film mounted on the recording drum point by point and line by line.
The exposed and developed films received are the desired color separations: magenta, cyan and yellow.
In order to calibrate the color correction and contrast adjustement for the reproduction apparatus, it is necessary that there be a measurable estimate of the properties of the film even before the film is exposed. An estimate of the film densities for a reference gray scale is supplied by the corrected color signals. The film density may, for example, be obtained by measuring the voltage of the color signal with a voltage meter which is calibrated for the range of the density values and placed at the output terminals of the color operator.
At this stage, the functional relationship between the film densities of the reference gray scale and the actual film densities on the exposed and developed film is not linear due to the non-linearities of both the film and the recording lamp.
The graphical representation of the non-linearities of the film is called film contrast or gradation and it will be identified herein as the characteristic film curve. Such a curve shows the non-linear relationship between the obtained film densities and the logarithm of the light quantity affecting the film. The general character of the film curve is influenced substantially by the properties of the film, by the developing process and its parameters, the composition and concentration of the developer itself, the developing time, and the developing temperature, as well as by the movement of the film during the developing and drying. In the event only one of the parameters changes, then the general shape of the characteristic film curve will also change.
By the employment of automatic developing devices in which the developing temperature is kept constant and the developer is regenerated and moved, even after a long operating time, a stabilization of the developing process is still possible. However, the stabilization, at least at the initial phase after longer periods of non-use, is unsatisfactory, because the shape of the characteristic film curve becomes changed. The characteristic film curve always is different for films having a different emulsion number or a different type of processing is used.
Generally, there is a non-linearity of a recording lamp between its brightness and the control current driving the lamp. In order to compensate for this non-linearity in the case of a known color scanner, a function generator is provided and is placed between the volt meter indicating the desired film density and the recording lamp, that is, in the path of the color component signals.
With such function generator, there is formed a compensating or correcting function having a polygonal shape in which the steepness or slope of the individual segments and the break in the line can be set by a plurality of regulators.
In order to set the correction or compensating function in a known color scanner, first the non-linearities have to be obtained. In such apparatus, a sample gray scale or wedge is exposed onto the component film or on a film having the same gradation value by feeding a plurality of voltage signals into the input of the recording unit. The obtained film densities of the individual gray shades of the sample gray wedge are measured by means of a densitometer and the measured values are recorded graphically as a curve as a function of voltage. After the curve has been obtained, the compensating function is established in the function generator.
To this effect, to the input terminals of the function generator, sequentially, different voltage values are fed and the voltage values are selected in such a manner, for example, that the densitometer coupled to the input terminals of the function generator will indicate sequentially the desired film densities of the reference gray wedge between 0.1 through 2 in steps of 0.1.
For each indicated film density, the associated voltage value is obtained from the recorded curve. If the recording lamp is supplied with such voltage values, then the obtained film density will correspond to the reference gray wedge film densities.
The contribution of the operating personnel will amount only to the setting up of a correction function with the help of the regulator of the function generator in such a manner that for each indicated film density, a corresponding voltage value will appear at the output of the function generator which has been obtained from the curve.
Then the correction function becomes approximated in such a manner that the non-linearity on the signal path of the color component signals become compensated.
Inasmuch as the regulators used will influence each other, the alignment process must be repeated several times.
The correction for non-linearities as mentioned above must be performed each time a different type of film is used or when the new film used has an emulsion number which is different from the previous one. In addition, in practice, a calibration must be made at least once a day in order to control the developing process. The alignment process takes about an hour.
There are scanners known in which only the density indication is made linear. For this reason, the function generator is arranged before the densitometer and the color component signals delivered from the color operator are directly fed to the recording lamp.
The above discussion shows that the frequency and the time spent on correcting non-linearities will result in the loss of a great deal of the useful time of the color scanner. The known method and processes are in addition not very accurate, inasmuch as the correction curve is constructed from only a few measured points.